add github build workflow

Pr181

Add isString () convenience method

Pi value

Update the project, if you like my changes

Add option to override duplicate settings

Fix compilation of tests for out-of-tree-build

Fixed project version escape character

Fix memory leak when parsing invalid syntax

Add a destructor for TOK_STRING that is called when a TOK_STRING is
discarded during error recovery.
Closes #142.

`lib/grammar.c` was regenerated using "bison (GNU Bison) 3.0.4".

libconfig rejects settings with duplicate names by default.
With this option enabled, libconfig will instead remove the old
setting and add the new one.

We have to modify the include path, otherwise I get

| libconfig/tests/tests.c:31:10: fatal error: libconfig.h: No such file or directory
|    31 | #include <libconfig.h>
|       |          ^~~~~~~~~~~~~
| compilation terminated.
Signed-off-by: Bernhard Walle <bernhard.walle@ncp-e.com>

Fix to undo #120 and make Docker compatible

Add missing include in CMakeLists

Add modern *Config.cmake

These files will be generated and installed:

* libconfigConfig.cmake
* libconfigConfigVersion.cmake
* libconfig++Config.cmake
* libconfig++ConfigVersion.cmake

Add the `include(CheckSymbolExists)` to allow the usage of the `check_symbol_exists()` macro and fix #136

Add the `include(CheckSymbolExists)` to allow the usage of the `check_symbol_exists()` macro and fix #136